Data storage media are commonly used for storage and retrieval of data and come in many forms, such as magnetic tape, magnetic disks, optical tape, optical disks, holographic disks or cards, and the like. In magnetic media, data is typically stored as magnetic signals that are magnetically recorded on the medium surface. The data stored on the medium is typically organized along “data tracks,” and transducer heads are positioned relative to the data tracks to read or write data on the tracks. A typical magnetic storage medium, such as magnetic tape, usually includes several data tracks. Optical media, holographic media and other media formats can also make use of data tracks.
As the number of data tracks on a medium increases, the data storage capacity of the medium likewise increases. However, as the number of data tracks increases, the tracks usually become narrower and more crowded on the medium surface. Accordingly, an increase in the number of data tracks can make positioning of the transducer head relative to a desired data track more challenging. In particular, for proper data storage and recovery, the transducer head must locate each data track, and follow the path of the data track accurately along the media surface. In order to facilitate precise positioning of the transducer head relative to the data tracks, servo techniques have been developed.
Servo patterns refer to signals or other recorded marks on the medium that are used for tracking purposes. In other words, servo patterns are recorded on the medium to provide reference points relative to the data tracks. A servo controller interprets a detected servo pattern and generates a position error signal (PES). The PES is used to adjust the lateral distance of the transducer head relative to the data tracks so that the transducer head is properly positioned along the data tracks for effective reading and/or writing of data to the data tracks.
With some data storage media, such as magnetic tape, the servo patterns are stored in specialized tracks on the medium, called “servo tracks.” Servo tracks serve as references for the servo controller. A plurality of servo tracks may be defined in a servo band. Some magnetic media include a plurality of servo bands, with data tracks being located between the servo bands.
The servo patterns recorded in the servo tracks may be sensed by one or more servo heads. Once the servo head locates a particular servo track, one or more data tracks can be located on the medium according to the data track's known displacement from the servo track. The servo controller receives detected servo signals from the servo heads and generates PESs, which are used to position a read/write head accurately relative to the data tracks.
Two general categories of servo patterns are amplitude-based servo patterns and time-based servo patterns. Amplitude-based servo patterns refer to servo patterns in which detection of magnetic servo signal amplitudes can enable identification of head position relative to the medium. Time-based servo techniques refer to servo techniques that make use of non-parallel servo marks and time variables or distance variables to identify head position.
Conventional amplitude-based servo patterns typically include servo “windows” adjacent to the servo tracks. The servo windows may comprise regions where a magnetic signal has been erased from the medium, and may be arranged in a checkerboard-like configuration to define a plurality of servo tracks. As a servo head passes relative to the medium, it detects a signal amplitude relative to a proportion of the servo head that passes over servo windows positioned along a servo track. In this manner, the precise location of servo tracks adjacent the servo windows can be identified. Amplitude-based servo patterns are commonly implemented in magnetic tape media, but may also be useful in other media.
When time-based servo techniques are used, the time offset between the detection of two or more servo marks can be translated into a PES, which defines a lateral distance of the transducer head relative to a data track. For example, given a constant velocity of magnetic tape formed with servo pattern “/\”, the time between detection of mark “/” and mark “\” becomes longer when the servo head is positioned towards the bottom of pattern “/\” and shorter if the servo head positioned towards the top of pattern “/\”. Given a constant velocity of magnetic media, a defined time period between detected servo signals may correspond to a center of pattern “/\”. By locating the center of pattern “/\”, a known distance between the center of the servo track and the data tracks can be identified. Time-based servo patterns are also commonly implemented in magnetic tape media, but may be useful in other media.